It has been found desirable to have a high-power index-guided diode laser operating in a wavelength-locked single longitudinal mode and with a well defined single lateral spatial mode. Such diode lasers have applications in space communications, optical recording and fiber-optics. To achieve such a diode laser, a channeled substrate planar (CSP) AlGaAs diode laser was developed. Such a diode laser has a channel in the substrate for lateral confinement and a clad layer over the substrate and filling the channel. The clad layer has a planar surface over which the active layer of the diode laser is grown epitaxially. This type of diode laser has all of the above desired properties except spectral stabilization.
In order to achieve both spectral and spatial stability, a channel substrate, planar, distributed. feedback diode laser was developed. Such a diode laser is described in an article entitled "Efficient AlGaAs channeled-substrate-planar-distributed feedback laser" by B. Goldstein et al., published in Applied Physics Letters, Vol. 53(7), Aug. 15, 1988, pages 550-552. This diode laser includes a grating to achieve spectral stability. The grating shown in this article is an interrupted grating in that it is positioned in the wings (shoulders) of the device on either side of a V-channel in the substrate. Although the structure displays distributed feedback characteristics and the desired spatial stabilization, the device exhibits distributed feedback behavior only over a limited temperature range (&lt;10.degree.C.) and the yield of the devices having these desired characteristics is very low (less than about 5%). The reason for this is that the optical mode only couples to the grating over the wings of the device and thereby provides a very low coupling coefficient, which describes the extent of the interaction between the optical field of the diode laser and the grating.
To enhance the interaction between the grating and the optical field, it has been suggested that the grating be positioned in the structure such that a greater portion of the optical mode interacts with the grating. Diode lasers having confined channels with a grating extending across the entire device are shown in U.S. Pat. No. 4,257,011 (M. Nakamura et al.), issued Mar. 17, 1981, and entitled "Semiconductor Laser Diode", in U.S. Pat. No. 5,027,368 (H. Kudo et al.), issued Jun. 25, 1991, and entitled "Semiconductor Laser Device" and in an article entitled "Continuous room-temperature operation of a 759-nm GaAlAs distributed feedback laser" by S. Takigawa et al., published in Applied Physics Letters, Vol. 51(20), Nov. 16, 1987, pages 1580-1581. Although this structure greatly increases the coupling efficiency, it also provides a large beam divergence of the emitted beam (as large as 50.degree.). Such a beam divergence is undesirable for most applications requiring coupling to optical systems. A smaller beam divergence in the range of about 25.degree. to 29.degree. allows for more efficient use of the light output from the diode laser with a simple optical system.
To achieve a smaller beam divergence, a large optical cavity, distributed feedback-channeled substrate planar (LOC-DFB-CSP) semiconductor laser was developed. This laser, which is described in U.S. Patent application Ser. No. 07/651,076 (John C. Connolly et al.), filed Feb. 6, 1991, entitled "Semiconductor Diode Laser" and assigned to the assignee of the present application, comprises a substrate of n-type conductivity gallium arsenide having a channel along one surface thereof between its end surfaces. A first clad layer of n-type aluminum gallium arsenide is on the surface of the substrate. The first clad layer fills the channel and has a planar surface. A thin active layer of undoped aluminum gallium arsenide is on the clad layer and a spacer layer of p-type conductivity aluminum gallium arsenide is on the active layer. A grating layer of p-type conductivity aluminum gallium arsenide is on the spacer layer and has a second order grating therein which extends across the channel in the substrate. A second clad layer of p-type conductivity aluminum gallium arsenide is on the grating layer and a cap layer of n-type conductivity gallium arsenide is on the second clad layer. A p-type conductivity contact region extends through the cap layer to the second contact layer and is over the channel in the substrate. Conductive contacts are on the contact region and a second surface of the substrate. The composition and thicknesses of the active layer, spacer layer and grating layer are such as to provide the output beam of the diode laser with a relatively low divergence angle of about 27.degree..
The Connolly et al. LOC-DFB-CSP semiconductor laser described hereinabove is made using two different growth methods. After the groove is formed in the surface of the substrate, the first clad layer, the active layer, the spacer layer and the grating layer are grown in succession on the substrate using liquid phase epitaxy. After the grating is formed in the grating layer, the second clad layer and the cap layer are grown in succession using metalorganic chemical vapor deposition (MOCVD) techniques. The LPE process is used for the initial layers since it fills and planarizes the V-channel in the substrate which provides lateral mode confinement. The MOCVD process is used for the final layers on top of the grating layer since this provides for greater ease of regrowth on aluminum gallium arsenide.
Although this semiconductor laser provides a beam having a narrow divergence angle of about 27.degree., it cannot achieve low threshold current operation for a beam having a divergence angle of less than about 36.degree.. Beam divergence decreases with increasing active layer thickness. A beam divergence of 30.degree. or less requires an active layer thickness of less than 0.06 micrometers. However, threshold current increased rapidly for active layers having a thickness of less than about 0.07 micrometers. Lasers with thin active layers have a small optical confinement factor, which leads to a high threshold current. Thus, a narrow beam divergence of &lt;27.degree. is incompatible with low threshold current operation. Also, the use of LPE to grow the active layer leads to problems with regard to threshold current. The LPE growth process has an intrinsic layer thickness variation which is exacerbated for layers thinner than about 0.07 micrometers. Thin layers have a measured thickness variation across a wafer of about plus or minus 10-15 nanometers, or a percent variation of plus or minus 20-30% for nominally 0.05 micrometer thick layers. This leads to a large variation in the threshold current density and in the grating strength of the device. A greater uniformity in grating strength and in threshold current could be obtained if the minimum layer thickness of the active layer was increased to 0.07-0.09 micrometers. However, a small beam divergence of less than 27.degree. cannot be simultaneously achieved for such layer thicknesses in the structure described above. Therefore, it would be desirable to have a DFB-CSP semiconductor laser which would have both a narrow beam divergence (less than 27.degree.) and a low threshold current.